1. Field of the Invention
This invention relates to a method and apparatus for heat-treating nonwoven webs.
2. Description of Related Art
Methods for drying and heat-treating sheet-type materials such as nonwoven, knitted, and woven fabrics are known in the art. For example, air-impingement and flotation dryers are known in the art that are suitable for drying sheet materials that can tolerate the relatively high tensions used to pull the sheet through the process. Porous sheet materials can also be heated while under low or substantially zero tension by pulling a heated gas, such as air, through the fabric while pinning the fabric to a porous surface such as a drum or belt. For example, in a through-air bonder hot air on one side of a sheet is pulled through the sheet by applying a vacuum to the opposite side, the vacuum also serving to pin the sheet to a porous surface on which the sheet is supported. It is also known to remove residual water from a nonwoven fabric that has been topically treated with a chemical finish composition by passing the fabric over steam cans.
An air levitation dryer developed by Mascoe for processing coated webs is described in Journal of Coated Fabrics. Vol. 25, January 1996, pp. 190-204. The levitation system is described as having the ability to support webs up to 60 inches wide and 50 feet in length without using a transport conveyor or tenter and with no contact to any supporting surface using no more than 10 pounds lineal tension. Such dryers have the limitation that they are not readily adapted to high processing speeds. Other low-tension dryers are disclosed in International Dyer, 185, Number 3, p. 27 (March, 2000). In one example, a fabric is transported in a tensionless state using a conveyor belt and overfeeding the fabric, alternating between sections where the fabric is run in a wave by means of alternate upper and lower air flows and sections where suction is performed under the belt.
However, known drying processes can cause defects to form in the nonwoven webs during the drying process, such as waves or puckers in the fabric sheet, especially when the polymeric fiber component(s) in the fabric are relatively low-melting temperature materials.
It would be advantageous to be able to dry and/or cure chemical finishing agents applied to a nonwoven web or sheet which comprises relatively low-melting temperature polymeric fiber components, or otherwise heat-treat such a nonwoven web or sheet, without creation of defects in the dried nonwoven sheet or web.
In a first embodiment, the invention is directed to a method for drying a nonwoven fabric that has applied thereon a chemical finish composition comprising the steps of providing a nonwoven fabric comprising thermoplastic polymeric fibers and containing a chemical finish composition comprising a solvent and at least one chemical agent; applying tension to the nonwoven fabric and transporting the fabric containing the chemical finish composition through a first drying zone wherein the solvent content of the nonwoven fabric is reduced to no less than about 2 weight percent, based on the dry weight of the nonwoven fabric, as the nonwoven fabric exits the first drying zone; transferring the nonwoven fabric from the first drying zone to a second drying zone, wherein the tension applied to the nonwoven fabric in the second drying zone is less than the tension applied to the nonwoven fabric in the first drying zone; heating the nonwoven fabric in the second drying zone to substantially completely remove the solvent from the nonwoven fabric; and cooling the nonwoven fabric in a cooling zone.
In another embodiment, the invention is directed to a method for heat treating a multiple component nonwoven fabric comprising a first polymeric component and a second polymeric component, the first polymeric component having a melting point or softening point that is lower than the melting point or softening point of the second polymeric component, comprising heating the nonwoven fabric to a temperature that is greater than about (Tmxe2x88x9240)xc2x0 C., where Tm is the melting or softening point of the first polymeric component, but less than about (Tmxe2x88x9210)xc2x0 C. while the nonwoven fabric is under a tension in any one direction that is between 0 and 52.5 N/m.
Another embodiment of the invention is directed to an apparatus for heat-treating a sheet material comprising a first heating zone; a second heating zone; and a tension isolation means disposed between the first and second heating zones, wherein the tension isolation means applies tension to the sheet as it is conveyed through the first heating zone and causes a reduction in tension on the sheet as the sheet exits the tension isolation means and is conveyed through the second heating zone.
Another embodiment of the invention is directed to a nonwoven fabric comprising fibers which comprise polyethylene, the nonwoven fabric having a chemical agent applied thereon and having a Frazier air permeability of at least 5 m3/min/m2 and characterized by less than 1.2 stretch-type defects/m2.